The present embodiments relate to a magnetic resonance (MR) device.
In MR tomography as practiced today, images having a high signal-to-noise ratio (SNR) are generally acquired using “local coils” (e.g., “loops”). In this case, the excited nuclei of a measurement object induce a voltage in the local coils. The induced voltage is amplified by a low-noise preamplifier (LNA) and finally forwarded at the MR frequency via a cable connection to a receiver (e.g., receive electronics) of an MR receive system of an MR device.
Powerful devices known as “high-field” systems are employed in order to improve the signal-to-noise ratio, including in the case of high-resolution images. The basic field strengths of such systems currently range up to 3 tesla (T) and higher. Since it is often possible to connect a greater number of local coils or loops to the MR receive system than the number of receivers present, a switching matrix (e.g., an “RCCS”) is installed between the local coils and the receivers. The switching matrix routes or leads the currently active receive channels (each having one or more local coils) to the available receivers. By this, a greater number of local coils than there are receivers present may be connected, since with whole-body coverage, only the local coils that are located in the Field of View (FoV) or covered visual field or in a homogeneity volume of the magnet may be read out.
A local coil may have one or more coil elements (e.g., antenna elements or receive antennas). For example, a local coil that includes a plurality of coil elements may be referred to as an “array coil”. A local coil may include the at least one coil element, the low-noise preamplifier, further electronics and cabling, an enclosure, and may also include a cable with plug connector by which the local coil is connected to the rest of the MR device. The MR device has, for example, at least one MR receive system.
The local coils are normally located in a transmit field (e.g., B1 magnetic field) of a whole-body coil or simply “body coil”. The body coil is a large coil that encircles an object that is to be measured, such as a body, and that is used to excite the spins in the measurement object. The body coil generates a circular or elliptical B1 magnetic field. The B1 magnetic field causes currents to be induced on all conductors that are located in the field. For example, on longer cables (e.g., embodied as coaxial cables), this may lead to resonance effects due to the cables acting as an antenna, causing high currents to flow on the outside surfaces of the cable sheaths. The high currents potentially pose a risk to the patient as a result of a buildup of heat. Long before the currents are so high that a risk of injury to the patient becomes relevant, the B1-induced currents may furthermore cause secondary B1 fields on the conductor structures of the cables that destroy a B1 homogeneity of the body coil transmit field.
A further problem with the use of a coaxial cable occurs if the coaxial cable experiences a discontinuity (e.g., if the coaxial cable is soldered onto a printed circuit board or the coaxial state is exited in some other way). In such a case, the preamplified receive signals may pass from the inside of the sheath to the outside. If the coaxial cable is routed close to and along the length of a coil element, the preamplified signal may feed back again into the local coil, thereby resulting in the self-oscillation of the associated local coil. In this state, the local coil may not be used for MR imaging.
For this reason, all line structures that are significantly longer than several tens of cm in high-field systems (e.g., of 1.5 T up to 3 T or more) are to be provided with standing wave traps (SWTs). The standing wave traps are resonant blocking circuits that suppress the flow of current in the range of the resonance frequency. In prior art MR systems, the transmit frequency of the body coil and the receive frequency of the local coils, as well as the signal that is transferred from the local coils on cables to the system, are identical, so in most cases, a solution is adopted for building a standing wave trap having two coils; one of the two coils connects an inner sheath surface of two sheath sections, and the other of the two coils connects an outer sheath surface of the two sheath sections and is grounded at both ends via a respective resistor. As a result, the standing wave current generated by the transmit or TX field is suppressed only on the outer surface of the sheath of the coaxial cable, but not on the inner surface thereof. A suppression on the inner surface is not permissible, because otherwise, the wanted signal is also suppressed. Although other designs of standing wave traps, such as the “bazooka” balun-type shield current cable trap, have a geometrically different structure, the standing wave traps are also aimed at suppressing only the current on the outer surface of the sheath and allowing the current on the inner surface to flow unimpeded at the MR frequency.
Standing wave trap chains or “SWT chains” in present-day use consist of sleeve baluns that have a number of individual parts (e.g., printed circuit boards, enclosures, screws, etc.), all of which require labor-intensive and time-consuming individual mounting. This provides that standing wave traps composed of L/C combinations may currently only be wound manually and are manually produced special parts made specifically for MR applications. This leads to a need for balancing (e.g., standing wave traps of contemporary design are not easily tunable due to the fact that the inductance is defined by the number of turns of the coax inductance) and results in very high costs.
The use of coaxial cables also leads to high costs due to the fact that coaxial cables are approximately 5-8 times more expensive than simple stranded wires or other solutions.
A very large number of standing wave traps connected in parallel diminish the barrier effect with respect to the standing wave because the parallel connection arrangement leads to a lower resistance with regard to the standing wave.
Because of the large diameter and small minimum bending radius of the coaxial cables, standing wave traps also have high space requirements (e.g., in order to accommodate the coaxial cable wound to form an inductance). The space requirement in local coils is a problem, for example, for local coils that are designed to be mechanically flexible.
U.S. Pat. No. 7,777,492 B2 discloses an arrangement for transmitting an informative signal generated by a suitable signal generator (e.g., from a first electrical site to a second electrical site). The first electrical site is connected to the second electrical site by a capacitively coupled transmission line. Capacitors configured in a distributed or lumped arrangement may be used in order to realize such a capacitively coupled transmission line. The arrangement may be connected to an add-on device, which may include a spectrometer, a further signal generator, a tuning device, etc. The further signal is generated by the add-on device and transported via the capacitively coupled transmission line in a direction from the second electrical site to the first electrical site. The further signal may be used for feeding an amplifier or for carrying the signal. The arrangement further relates to a magnetic resonance compatible device, a magnetic resonance imaging system, and a method for sensing magnetic resonance energy.
DE 10 2010 012 393 A1 or U.S. Pat. No. 8,547,098 B2 discloses a magnetic resonance system including at least one coil and at least one coaxial line connecting the coil to an electronic receive system. The coil has a preprocessing device for converting the received signals to at least one transmission frequency that is different from the transmit frequency. The coaxial line has at least one standing wave trap having a trap circuit for suppressing sheath waves of the transmit frequency both on the outer surface and on the inner surface of the sheath conductor of the coaxial line. These two documents are aimed at the use of standing wave traps in the inner and outer conductor of coaxial cables that allow intermediate frequencies to pass through, but have a barrier effect with regard to the Larmor frequency.
DE 10 2010 031 933 A1 or US 2012/0187950 A1 discloses that a printed circuit board is embodied as flat (e.g., having a top side and a bottom side, each of which is delimited by long and short lateral edges). Narrow sides extend from the top side to the bottom side on the long lateral edges, and end faces extend from the top side to the bottom side on the short lateral edges. A number of terminations for shielded cables are arranged on the top side in proximity to one of the end faces. Each of the terminations has at least one contact that is connected to a respective conductor track of the printed circuit board that, starting from the respective contact, extends in the direction of the other end face. The respective conductor track is connected either to a respective local coil for magnetic resonance applications that is arranged on the printed circuit board or to a contact of a respective further termination for a respective further shielded cable that is arranged on the top side or on the bottom side. The local coil and/or at least one of the further terminations is arranged in proximity to the other end face. The conductor tracks run in at least one intermediate layer of the printed circuit board that is arranged between the top side and the bottom side. A basic shield that is impervious to frequencies in the magnetic resonance range is arranged on the top side and/or on the bottom side, and an auxiliary shield that is impervious to frequencies in the magnetic resonance range and is electrically connected to the basic shield is disposed on the narrow sides. These two documents are aimed at replacing a cable harness having standing wave traps in the form of sleeve baluns by an elongate printed circuit board and at routing the line in the interior of an RF shield. The flow of current at the MR frequency is suppressed on the shield by sleeve baluns.